The present invention relates to a circuit used to simulate the electrical characteristics of an inductor, and more particularly, to utilize the inductor equivalent circuit to a tunable band pass filter and an oscillator circuit.
An analog circuit usually includes both active device and passive device. The former is typically a transistor, and the latter is usually composed of capacitor, inductor, and resistor. A filter having only components of one or more inductors, capacitors and resistors is called a passive filter. In recent decade, thin film semiconductor techniques for manufacturing the capacitor and the resistor have gained a great achievement. For example, integrating several millions of resistors, capacitors, and transistors in a chip having size of about a thumb isn""t longer a difficult matter exclude inductors. To make a inductor without occupying large planar area by semiconductor integrated circuit (IC) is difficult. The inductor formed is usually by means of wire winding with very long metal wire for an inductor in reasonable inductance value so as to suit for applications and thus cause it impractical to integrate the inductors in IC chips for most applications.
Fortunately, active type filter has been developed recently. The circuit is composed of operational amplifiers, transistors, resistors, and capacitors without inductors. As a result, the filter can be shrinkage in size and integrated in IC chips. The active type filter is thus more suitable than the passive type filter to be built inside IC""s. However, the characteristic of the operational amplifier itself restricted the frequency band width and the power consumption. Consequently, the active type filter is usually used in a range of lower frequency processing, for instance, audio band and below.
Liu, in U.S. Pat. No. 5,600,288 with a title xe2x80x9cSynthetic Inductor in Integrated Circuits for Small Signal Processingxe2x80x9d proposed a RLC circuit composed of a current mirror circuit incorporated with a plurality of transistors, a resistor and a capacitor to successively simulate an inductor. According to the circuit, inductances can be fabricated in a range between 100 xcexcH to 100 mH and having a frequency response available to great 10 MHz.
The present invention is to provide another types of simulated inductor circuit, particularity using in band pass filter, for which the center frequency is tunable in a very easy way, furthermore, it can be easily cascaded to form high order filters, keeping the center frequency tunable and consistent.
The invention discloses an inductor equivalent circuit and its application in active filter. Five preferred embodiments are proposed.
In the first embodiment, the circuit comprises a reference current source, a current mirror, two operational amplifiers OP1 and OP2, a capacitor, a first transistor, a second transistor, a first resistor, a third transistor, and a current source in parallel with the capacitor. Two resistors have one common terminal grounded and other terminals are, respectively, coupled with the two transistors. An input signal is through OP1 and second transistor. The current mirror is then feed-back a signal to the first transistor through OP2. The signal makes the drain current of the first transistor lags the input voltage signal by 90xc2x0 due to the capacitor coupled with the mirrored current source.
In the second preferred embodiment includes an extra terminal, the second mirror current, to provide an output current of the inductor equivalent circuit for a next cascade stage.
In the third preferred embodiment, the circuit comprises elements as in the second preferred embodiment, and an additional resistor which is coupled with the capacitor and ground to stabilize the circuit and also for applications with lower Q-values.
In the fourth preferred embodiment, the circuit is a simplify version of the circuit in the third preferred embodiment by skip OP1 and OP2. The resulting characteristics comparing with the circuit in the third preferred embodiment is slightly inferior in distortion, but having a wider applicable frequency range.
In the fifth preferred embodiment, all the resistors are replaced with a resistor mirror set, a plurality of transistors, with gates terminal thereof coupled with a biased signal. The biased signal is a proper voltage so that the resistor mirror set consisting of transistors is operated in an ohmic region. The gate bias of the transistors of the resistor mirror set can be modulated to set the equivalent resistance value of the transistors, thus to adjust the center frequency of the filter. Furthermore, a resistor mirror circuitry is also proposed to mirror the desired resistance value to the transistors of the resistor mirror set.